This invention relates to thermographic recording elements and more particularly, to photothermographic recording elements suitable for the manufacture of printing plates.
Photothermographic materials which are processed by a thermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d in xe2x80x9cImaging Processes and Materials,xe2x80x9d Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tone of silver, and a reducing agent, typically dispersed in a binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80xc2x0 C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming images.
Such photothermographic materials have been used as microphotographic and medical photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has so high sensitivity and Dmax that it may comply with such output devices.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the graphic printing field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic materials for use in the graphic printing field which can be effectively exposed by means of laser image setters and produce clear black images having a high resolution and sharpness. These photothermographic materials offer to the customer a simple thermographic system which eliminates a need for solution type chemical agents and is not detrimental to the environment.
U.S. Pat. No. 3,667,958 discloses that a photothermographic element comprising a polyhydroxybenzene combined with a hydroxylamine, reductone or hydrazine has high image quality discrimination and resolution. This combination of reducing agents, however, was found to incur an increase of fog.
For producing a thermographic recording element having high Dmax and high contrast, it is effective to add to the element the hydrazine derivatives described in U.S. Pat. No. 5,496,695. Although this results in a thermographic recording element having high Dmax and high contrast, all of sensitivity, contrast, Dmax, Dmin, and storage stability of compounds are not fully satisfied.
Improvements in contrast and storage stability of compounds are achieved by using the hydrazine derivatives described in EP 762196A1, but the fully satisfactory level has not been reached.
Further, U.S. Pat. Nos. 5,545,515 and 5,635,339 disclose the use of acrylonitriles as the co-developer. With these acrylonitrile compounds, a fully satisfactory high contrast is not achieved, fog rises, and the photographic properties largely depend on the developing time.
An object of the present invention is to provide a thermographic recording element having low fog, high sensitivity, high Dmax, and minimized developing-temperature dependency.
Another object of the present invention is to provide a recording element for use in the manufacture of graphic printing plates which forms an image of quality and can be processed in a fully dry basis without a need for wet processing.
According to the invention, there is provided a thermographic recording element having at least one image forming layer. The element contains an organic silver salt, a reducing agent, and at least one of compounds of the following formulas (A) and (B). 
Herein, Z1 and Z2 each are a group of non-metallic atoms capable of forming a 5-7-membered ring structure with the carbon atoms; Y1 and Y2 each are xe2x80x94C(xe2x95x90O)xe2x80x94 or xe2x80x94S2Oxe2x80x94; X1 and X2 each are a hydroxy or salt thereof, alkoxy, aryloxy, heterocyclic oxy, mercapto or salt thereof, alkylthio, arylthio, heterocyclic thio, amino, alkylamino, arylamino, heterocyclic amino, acylamino, sulfonamide or heterocyclic group; and Y3 is hydrogen or a substituent.
Preferably, the compound of formula (A) has at least 6 carbon atoms in total, and the compound of formula (B) has at least 12 carbon atoms in total. More preferably, in formula (A), the total number of carbon atoms in Z1 is at least 3, and in formula (B), the total number of carbon atoms in Z2 and Y3 is at least 8.
In one preferred embodiment wherein a photosensitive silver halide is further contained, a photothermographic recording element is provided.